I rarely see Black Widow spiders, but I know they are around. I see their webs, unkept, straglely messes than one might carelessly assume are abandoned. Wikipedia describes it this way: ". . . the widow spiders construct a web of irregular, tangled, sticky silken fibers. . ."
If you look closely or click on the first photo to enlarge it, you can vaguely see the web around the bottom of the gas can.
The Black Widows have poor eyesight, are timid, are less-than-deadly poisonous, bit mostly defensively, and tend to live in shadows, often spending daytime in their lair and only tending web at night.
Crawling the Internet, I found an interesting Black Widow utube video: